1. Field of the Invention
This invention relates to an electrical device, particularly to a device to be used in digital telecommunications applications, and to assemblies of electrical devices.
2. Introduction to the Invention
Circuit protection devices are well known. Those circuit protection devices which are particularly useful in some applications, e.g. to protect telecommunications circuits, exhibit positive temperature coefficient of resistance (PTC) behavior, i.e. the resistance increases anomalously from a low resistance, low temperature state to a high resistance, high temperature state at a particular temperature, i.e. the switching temperature TS. Under normal operating conditions, a circuit protection device which is placed in series with a load in an electrical circuit has a relatively low resistance and low temperature. If, however, a fault occurs, e.g. due to excessive current in the circuit or a condition which induces excessive heat generation within the device, the device “trips”, i.e. is converted to its high resistance, high temperature state. As a result, the current in the circuit is dramatically reduced and other components are protected. When the fault condition and the power are removed, the device resets, i.e. returns to its low resistance, low temperature condition. Fault conditions may be the result of a short circuit, the introduction of additional power to the circuit, power surges, or overheating of the device by an external heat source, among other reasons. When the device comprises a conductive polymer composition, during the tripping event the device expands as the polymer melts.
Devices intended for use in protecting telecommunications circuits and equipment have special requirements. For example, it is important that the device be tripped by the fault conditions which occur when a power line, i.e. an electrical cable which carries high voltages (e.g. 250 to 600 volts), comes into contact with a telephone line. These fault conditions are often referred to as “power cross”. An accepted test for devices which will provide such protection is described in Underwriter's Laboratory Standard 1950 3rd edition, the disclosure of which is incorporated herein by reference. In this test, a device is subjected to an electrical cycle consisting of 600 volts AC and 40 A (short circuit) conditions, with a wiring simulator connected in series with the device. Under such test conditions, it is possible for the device to arc or flashover at the edge from one electrode to the other due to the high power levels. Simultaneously, the device is expanding rapidly in order to absorb the energy associated with the fault condition.
For certain applications, such as for equipment to be used in telephone network circuitry, components to be used in that equipment must meet additional requirements. For example, it is often required that a device meet the applicable tests as put forth in Bellcore GR-1089 specification for Electromagnetic Compatibility and Electrical Safety, the disclosure of which is incorporated herein by reference. One aspect of Bellcore GR-1089 is that a component must survive after exposure to high voltage, high current transients, meant to simulate lightning strikes.
Telephony systems are rapidly evolving due to the increased demand for high speed transmission of large amounts of data which is in the form of digital signals. Devices used in digital telecommunications circuits face requirements which are different from those conventionally required by analog and voice systems. At present, PTC devices with resistances greater than 20 ohms are used in a wide variety of telecommunications systems. For example the Raychem PolySwitch™ TR600-150 device is utilized in telecom applications and can have an installed resistance as high as 22 ohms (see the Raychem Circuit Protection Databook, October 1998, the disclosure of which is incorporated herein by reference). However, with digital circuitry the device resistance must be substantially lower than 20 ohms to minimize the loss of signal and distortion of the signal in the circuit. A typical application might involve the use of one PTC device in the tip section of a telecommunications circuit, and a second PTC device in the ring section. The relative resistances of these two devices must be stable to achieve an optimum signal-to-noise ratio. The device capacitance must be low to allow the transmission without distortion of high bandwidth signals typical of digital information streams. Miniaturization of digital devices requires that the components also be reduced in size, particularly in their “footprint” (i.e. space they require on a circuit board), and in their height off the board (i.e. the distance from the top of the device, including any insulating layers that are present, to the board), so that boards can be mounted into equipment at higher densities. Despite these size, resistance, and capacitance requirements, the device must pass the appropriate tests as outlined above, such as power cross test requirements as specified in UL1950, and lightning surge requirements as put forth in Bellcore 1089.
Use of electrically insulating coatings or housings to surround circuit protection devices and other electrical components is known. See, for example, U.S. Pat. Nos. 4,223,177 (Nakamura), 4,315,237 (Middleman et al), 4,481,498 (McTavish et al), 4,873,507 (Antonas), and 5,210,517 (Abe), the disclosures of which are incorporated herein by reference. Such coatings provide electrical insulation and mechanical protection, and are particularly important for use with devices exposed to high voltage conditions in which arcing from one electrode to the other may occur. However, some conventional coatings, e.g. epoxies, can restrict the expansion of the PTC element, causing the device to fail. Other conventional coatings, which are flexible or conformable, may crack or pull away from the device as a result of the expansion during tripping, leaving the device edges exposed and subject to further arcing. In addition, for a surface mountable device, the coating must remain intact and retain its functionality for inhibiting arcing from one electrode to another after installation on the board, which commonly involves a reflow operation in which solder is heated above its melting temperature.
Existing circuit protection devices for high voltage digital applications have relatively large footprints. For example, devices sold under the tradename PolySwitch® TR600-160 by Raychem Circuit Protection, a division of Tyco Electronics Corporation, meet the requirements of the UL1950 600 VAC, 40 A power cross test as outlined above, and are radial leaded devices which are approximately 6 mm wide. PolySwitch® TS600-200 devices, which are surface mountable, are over 8 mm wide. Because telecommunications equipment such as line cards typically incorporates multiple lines, and each line is usually protected by its own separate circuit protection element, circuit boards often have eight or sixteen such devices mounted as close together as possible. Therefore, reduction in footprint for an individual device provides a multiple benefit for a circuit board. Further reduction in footprint can be achieved by packaging multiple devices together in an assembly.
For digital telecommunications circuits, it is important that the resistance of the series circuit protection elements be reduced to as low a value as possible, while still performing their intended function of protecting against various types of electrical faults. Most speech energy has been determined to be in the frequency range below 3500 Hz. The standard “4 kHz” voice channel universally used in telephone networks is designed to pass frequencies in the range 300-3400 Hz. Analog equipment such as a modem must force data to fit into this channel width by using various techniques, e.g. modulation, to overcome the bandwidth limitations of the telephone channel. However, digital services can provide much higher bandwidths. Digital systems include HDSL (high speed digital subscriber line), which operates at speeds up to 1.5 Mb/s (megabits per second), ADSL (asynchronous digital subscriber line), which operates at download speeds of up to 6 Mb/s and VDSL (very high speed digital subscriber line), which operates at up to 52 Mb/s. Higher speed systems exist for certain applications as well. Copper wire has a certain amount of resistance/length, and signals fade with distance. With amplifiers, the signal can be regenerated to some extent. However, with each amplification of signal, more noise is generated so after a point the use of amplification to transmit quality signals is limited. Any unnecessary resistance directly subtracts from the range the signal can be transmitted, and the quality of the signal. The impedance of the circuit is especially critical for the increased bandwidth requirements for high speed digital transmissions. Impedance mismatches can cause unwanted reflections in the circuits, and other sources of noise. Therefore it is important that impedance balance throughout the circuit be carefully designed and retained as the system is manufactured and operated in the field. Cross talk between lines further limits performance. All of these factors are extremely important in designing and optimizing digital systems, as signal-to-noise becomes a limiting factor in determining the range of the various digital architectures.
One method for reducing the resistance of PTC devices is to make the device larger. However, this approach can produce two deleterious effects. The first is clearly defeating the requirement that the device be as small as possible. For instance, board-to-board spacing in a piece of equipment may be 12.7 mm (0.5 inch), and therefore any device which extends beyond this distance could not be used. The second is that the device has an undesired large thermal mass which can be difficult to solder and may also not meet testing requirements. Besides the high voltage, high current power cross faults, circuit protection devices must also protect equipment against high voltage, low current faults. If the thermal mass of the device is too large, it will not trip under these conditions, where the fault current may be as low as 0.5 A, thereby exposing equipment to failure by a longer term lower energy fault condition.
Ceramic PTC devices have been used as circuit protection elements in telecommunication applications. However, because of the relatively high resistivity of ceramic materials, devices of low resistance will be undesirably large. In addition, the capacitance of the inorganic ceramic devices can be high, on the order of 1 nF, which is undesirable for high speed digital applications. Fuses remain an option for overcurrent protection for some applications; however, they are not resettable which can require undesired repair or replacement of equipment, which is often located in multiple or remote locations, at the manufacturer's cost.